Interpretive Summary: In June 2012, the orange rust of sugarcane was observed for the first time in Louisiana. The disease was observed in Florida in 2007 and was the first reported occurrence in the Western hemisphere. Subsequently, orange rust has been reported in most sugarcane production areas of Central and South America and the Caribbean. Sugarcane researchers in Florida have documented economic loss among susceptible sugarcane varieties. In the 3 months following the detection of orange rust in Louisiana, observations of orange rust have been limited to a recently released variety, Ho 05-961. Increase plots of this variety were surveyed, and orange rust symptoms were detected at 17 of 38 (45%) fields. The incidence and severity of the disease remains low, and the distribution appears to be limited to the southern portion of the Louisiana sugarcane production area. However, because the variety is susceptible to orange rust, HoCP 05-961 has been removed from the list of recommended for future planting.

Technical Abstract:
In June 2012, lesions typical of rust disease were observed on sugarcane cultivar Ho 05-961 (a complex hybrid of Saccharum L. species) on a farm near Schriever, Louisiana. Incidence and severity of disease symptoms were low. Two types of pustules were observed on leaves of the infected plants: urediniospores from one pustule type were reddish-brown in color, characteristic of brown rust which has been observed in Louisiana since 1979 (2), whereas urediniospores erupting from the other pustule type were orange in color. Samples of the two pustule types were collected. The morphology of the reddish-brown urediniospores was consistent with that described for Puccinia melanocephala Syd. & Syd., the fungus that causes brown rust of sugarcane, while the morphology of the orange urediniospores was consistent with those described for P. kuehnii E.J. Butler, the cause of orange rust of sugarcane (3). Telia and teliospores were not observed. The identity of the two Puccinia species causing the brown and orange rust lesions was verified using the species-specific quantitative PCR assays (1). Two DNA samples extracted from the pustules identified as P. kuehnii were independently subjected to PCR amplification using primers Pk1F and Pk1R (1) to yield a product from the rDNA which was then bidirectionally sequenced using the same primers. The resulting 480 nt sequences were identical to each other, and a BLAST search of Genbank revealed 100% identity to 19 previously reported isolates of P. kuehnii but not more than 89% similarity to any isolate of P. melanocephala (Virtudazo et al., 2001). This is the first report of orange rust in Louisiana. In the 3 months following the detection of orange rust in Louisiana, observations of the disease have been limited to Ho 05-961. Increase plots of this newly released cultivar were surveyed, and orange rust symptoms and urediniospores were detected at 17 of 38 (45%) fields. The incidence and severity of the disease remains low, and the distribution appears to be limited to the southern portion of the Louisiana sugarcane production area.